Modulation of chromatin structure plays an important role in transcriptional regulation. Much of this modification occurs at the level of the nucleosome, whose structure inhibits binding and function of numerous nuclear proteins. Nucleosomes are composed of a histone protein core surrounded by 147 base pairs of DNA. Nucleosome structure can be modified covalently, by complexes such as acetyltransferases or de-acetylases, or non-covalently via interactions with chromatin modifying complexes. These complexes can alter chromatin to create structures that are either permissive for factor binding and function ("activated") or non-permissive for factor binding and function ("repressed"). These complexes also play a role in maintaining gene expression patterns during development, and it has been proposed that they do this by establishing activated or repressed chromatin states that are maintained across cell division. The purpose of this application is to study in detail the mechanism of action of ATP-dependent chromatin remodeling complexes. This large and diverse group of complexes uses the energy of ATP hydrolysis to modify chromatin structure non-covalently by altering either histone-DNA contacts, histone-histone contacts, or both. The investigator works primarily with the human SWI/SNF family of remodeling complexes. Experiments are proposed that will: 1) investigate the energetics of SWI/SNF function, with the goal o developing an explicit model for how the energy of ATP hydrolysis is used to remodel nucleosome structure; 2) use mutagenesis and crosslinking to characterize the structural changes that occur to the nucleosome upon remodeling, and to characterize the portions of the histones that are required for these structural changes; and 3) compare and contrast the function of different members of the SWI/SNF family of complexes with each other and with the ISWI family of ATP-dependent remodeling complexes.